There is also another reason. At some point, and it's a sure bet with you guys, your'e gonna change or remove the barrel. If the lug is bedded too tight, it does not allow for slight rotational differences in the way the lug is located. If you've ever noticed when you snug up a barrel nut, the lug will rotate slightly. If the lug is not in the exact spot it was when it was bedded, it will cause the barrel to lean one way or the other. That's why you should have ample clearance all the way around the sides and the bottom. If you don't make clearance on the front side of the lug, it will shave bedding off every time you re-install the stock. This will leave little crumbs of bedding under the lug.
When ever I bed a full build, I use an oversize lug just for bedding purposes that is .025" bigger all the way around and has built in draft for easy removal.